Plant and Soil 191: 213–223, 1997. 213 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Trophic interactions between bacterial-feeding nematodes in plant rhizospheres and the nematophagous fungus Hirsutella rhossiliensis to suppress Heterodera schachtii R.C. Venette 1 , F.A.M. Mostafa 2 and H. Ferris 1 1 Department of Nematology, University of California, Davis, CA 95616, USA and 2 Faculty of Agriculture, Agricultural Zoology Department, Mansoura University, Mansoura, Egypt Received 10 September 1996. Accepted in revised form 19 March 1997 Key words: bacterial-feeding nematodes, cover crops, food webs, nematophagous fungus, suppressiveness, sus- ceptibility Abstract Trophic exchanges in soil food webs may suppress populations of pest organisms. We hypothesize that the sup- pressive condition of soils might be enhanced by manipulating components of the food web. Specifically, by enhancing populations of bacterial-feeding nematodes, propagule density of the nematophagous fungus Hirsutella rhossiliensis should increase and constrain populations of Heterodera schachtii, a plant-parasitic nematode. The rhizospheres of Crotalaria juncea and Vicia villosa stimulated population growth of the bacterial-feeding nematode, Acrobeloides bodenheimeri, but not of the nematodes Caenorhabditis elegans or Rhabditis cucumeris. The rhizo- spheres of Tagetes patula, Eragrostis curvula, and Sesamum indicum had no effect on any of the bacterial-feeding nematodes investigated. Acrobeloides bodenheimeri was most susceptible to parasitism by the nematophagous fungus H. rhossiliensis with 35% of individuals being parasitized in a laboratory assay. In three separate trials, par- asitism of H. schachtii by H. rhossiliensis was not enhanced when populations of A. bodenheimeri were amplified in a suitable rhizosphere. Introduction Suppressive soils occur where biotic and/or abiotic fac- tors restrict the growth and abundance of soil-borne plant pathogens (Schneider, 1982). Biotic factors con- strain pest densities through parasitism, predation, antibiosis, or competition. The degree of regulation depends on the density and activity of the biological agent and/or its target. In some cases, although antag- onists of plant pathogens may be present in a soil, the number and distribution of the antagonists may be insufficient to constrain the target pathogen. The distribution and abundance of predators and parasites is limited, to a large extent, by the availabil- ity of suitable prey and hosts (i.e., food). Oscillations in the number of antagonists may arise from predator- prey (Begon and Mortimer, 1986) or host-parasite FAX No.: 19167525809. E-mail: RCVENETTE@UCDAVIS.EDU interactions (Hassell and May, 1989) and may result in an inconsistent degree of suppression of the pathogen. For example, by the time antagonists respond numeri- cally to increased food availability, populations of the target already may have caused severe plant damage and economic loss. Thus, a strategy for more sustain- able pest control in agriculture is to provide a relatively consistent degree of plant protection by maintaining sufficient populations of antagonists. Bacteria, fungi, and animal predators that are antag- onistic to nematodes may protect plants from para- sitic nematodes (Duddington, 1957; Giuma and Cooke, 1974; Linford, 1937; Oostenbrink, 1960). These antag- onists may respond numerically to densities of their hosts, although the extent to which many of these antagonists rely on nematodes is unclear as certain fun- gal antagonists grow saprophytically (Cooke, 1962). For instance, Nicolay and Sikora (1991) found that the proportion of eggs of Heterodera schachtii that were